1. Field of the Invention
The present invention relates to a transfer unit of an electrophotographic printer, and more particularly, to a transfer unit of an electrophotographic printer in which a fuser roller and a transfer roller which print an image on a paper while pressing and passing the paper therebetween have improved structures.
2. Description of the Related Art
In general, an electrophotographic printer, such as a color laser printer, is provided with developing units for developing an electrostatic latent image formed on a photosensitive belt with a liquid developer, and a transfer unit for transferring the developed image on the photosensitive belt onto a paper. A drying unit evaporates and dries the liquid solvent of the liquid developer remaining on the photosensitive belt.
The transfer unit includes a transfer roller to which the developed image is transferred from the photosensitive belt and which prints the image on the contacting surface of the paper, and a fuser roller for pressing the paper toward the transfer roller. Therefore, the paper fed to the transfer unit passes through between the transfer roller and the fuser roller, i.e., a gap between the rollers, while being pressed by the rollers, and the image is printed on one surface of the paper.
However, since the transfer roller and the fuser roller have flat horizontal profile lines, the rollers bend when a paper is positioned between the rollers due to the pressing force between the rollers. That is, since the gap between the end portions of the rollers to which more pressing force is applied becomes relatively narrow, and the gap between the middle portion of the rollers becomes relatively wide, uniform pressure cannot be applied over the whole range of the paper. Of course, though the two rollers are in contact with each other and there is no substantially sensible gap between the rollers, the end portions of the rollers press the paper with a stronger force than the middle portions when the rollers bend.
In connection with the bending of the rollers as described above, rollers having special profiles have been proposed. The middle portion of a fuser roller may be is slightly bulged in comparison with the end portions. Therefore, the gap between the two rollers and can be horizontally straight and uniform when the rollers bend due to the pressing force therebetween. However, since there are linear velocity differences between the middle portion and end portions of the fuser roller when the roller rotates, the paper passing through between the two rollers and may be wrinkled or torn due to the linear velocity differences.
On the other hand, a fuser roller and a transfer roller having different profiles have been proposed. The fuser roller is formed to have a crowned profile in which the middle portion of the fuser roller is convex, and the transfer roller is formed to have a flared end profile in which the middle portion of the transfer roller is concave. That is, the fuser roller has a crowned shape, and the transfer roller has flared shapes at both ends. Therefore, the structures of the rollers as described above allow the widthwise gap by the pressing force between the two rollers to be uniform to some extent, and the difference in the linear velocities to be compensated for.
However, when a paper passes between the crowned roller and the flared roller, the result is an upward curl in which both sides of the paper are bent upward according to the shape of a nip between the rollers. In this case, since both sides of the printing paper are deformed in a rolled manner, there is an inconvenience in which the rolled paper must be flattened.